Harold Kestenholz
You are being advised well.
An added issue may be that you have the zones. When zone valves are closed, the full flow of water is not available, so water might not be flowing through the boiler fast enough to prevent local boiling. the collapsing of steam bubbles as they hit cooler water above in the piping can create rumbling. You might need a boiler bypass installed to maintain a minimum flow rate.
